Conventionally, there is an image forming apparatus (so-called wet type image forming apparatus) such as a printer and a copying machine in which the liquid developer including a carrier liquid and toner is used to form an image by an electrophotographic process. Particularly, in an image forming apparatus compatible with a color image, toner image formed on a plural photosensitive bodies which are of image bearing members are superimposed on an endless belt which is of an intermediate transfer body (first transfer) to form a full color toner image. Next, the full color toner image is transferred via a second transfer to paper which is of a recording medium.
In the image forming apparatus in which the intermediate transfer body is used, a second transfer roller press contacts the intermediate transfer body so that a nip is formed between the second transfer roller and the intermediate transfer body. The paper enters the nip while a second transfer bias voltage (current) having an opposite polarity to a polarity of the toner is applied to the second transfer roller, and the toner is sucked into the paper by an electrostatic force to perform the second transfer. On the other hand, except for the time of the second transfer, a reverse transfer bias having an opposite polarity to the second transfer bias is applied to the second transfer roller such that the residual toner on the intermediate transfer body are not attracted to the second transfer roller. When the toner adheres to the second transfer roller, the paper is contaminated. Additionally, the toner is fixed to the second transfer roller because the second transfer roller press contacts the intermediate transfer belt. As a result, the second transfer roller is unevenly charged, which possibly causes a second transfer defect.
In the wet type image forming apparatus, usually the image is formed using a carrier liquid which is of oil in nature (for example, silicone oil). Therefore, the carrier liquid adheres to the intermediate transfer body or the second transfer roller, and the paper twists easily around the intermediate transfer body or the second transfer roller due to the high viscosity of the carrier liquid, which sometimes causes a defect in a paper conveyer.
For example, Japanese patent Laid-Open Publication No. 2002-296926 relates to an image forming apparatus which prevents the adhesion of the liquid developer to a transfer member (second transfer roller) and re-adhesion of the liquid developer to an intermediate transfer body or a recoding medium. Specifically, the image forming apparatus includes a developer bearing member which bears the liquid developer in which the toner is dispersed into the carrier liquid, an image bearing member which bears a developed image, an intermediate transfer body which first transfers the developed image on the image bearing member, and a second transfer part for second transferring the developed image transferred onto the intermediate transfer body to the recoding medium. Additionally, the second transfer part includes a transfer member and a cleaning member, and the cleaning member is formed by an elastic body for cleaning a surface of the transfer member. In the image forming apparatus field, enhancement of an image forming speed is desirable and in demand, and this high-speed image formation is one of many factors for consideration when a user selects an image forming apparatus. However, the enhancement of an image forming speed means that a paper conveyer speed is enhanced. That is, a time period for the second transfer is shortened.
Unfortunately, in the case of an image having a small margin, the second transfer is not performed in a leading end portion (i.e., head portion of a page) of the image, and sometimes a portion to be printed is lacked or thinned (see FIG. 9B). The lack of the leading end portion is caused by performing the insufficient second transfer. That is, although a certain period of time (transition time) is required for a bias voltage (current) to reach a value necessary for the second transfer from a reverse transfer bias state, the toner image reaches, due to the speed enhancement, a position at which the toner image contacts the paper before the bias voltage reaches the value necessary for the second transfer.
Therefore, when the image has a narrow margin, usually a timing for starting electric polarity reversal from the reverse transfer bias to the transfer bias is set ahead. Herewith, when the toner image enters a second transfer nip, the second transfer bias voltage (current) is applied such that the second transfer roller reaches the value necessary for the second transfer. The second transfer transfers the image onto the paper without lacking the top portion of the image. But, the paper has a static charge because it is in contact with another paper and guide members of the conveyance path. Therefore, if the timing for starting electric polarity reversal is set ahead, when the sheet enters the second transfer nip, the relatively great bias is already applied to the second transfer roller. Therefore, the sheet is easily electrically attracted to the second transfer roller or the intermediate transfer body and twists around them. Furthermore, in the wet-type image forming apparatus, because of the high viscosity of the carrier liquid the sheet tends to twist around the second transfer roller and the like. Especially, there are paper jam problems that tend to occur at the second transfer nip by a combination of the high voltage and high viscosity when the timing for starting electric polarity reversal from the reverse transfer bias to the transfer bias is set ahead in the wet-type image forming apparatus.
In the image forming apparatus disclosed in Japanese patent Laid-Open Publication No. 2002-296926, in the case where the image forming speed is enhanced, and in the case where the margin next to the image is small, the problem of lacking the leading end portion of the image can be solved only when the timing for starting polarity reversal is set ahead. In that case, in the image forming apparatus disclosed in Japanese patent Laid-Open Publication No. 2002-296926, the paper jam often happens because of the paper twisting around the second transfer roller and the like and the above mentioned problem cannot be solved.
In the image forming apparatus disclosed in Japanese patent Laid-Open Publication No. 2002-296926, although the toner or carrier liquid adhering to the second transfer roller can be removed to some extent by the existence of a cleaning member, the fundamental problem of the easy contamination of the second transfer roller is not solved when the timing for starting the polarity reversal of the second transfer bias is set ahead. The cleaning member does not completely remove the contamination of the second transfer roller. In consideration of early abrasion of the cleaning member, the paper twists easily around the intermediate transfer body by the combination of the electrostatic force and the high viscosity of the carrier liquid.
In view of the foregoing, one object of the present invention is to provide an image forming apparatus which can properly perform the second transfer without setting ahead the timing for starting the polarity reversal from the reverse transfer bias to the transfer bias. Another object of present invention is to provide an image forming apparatus which could eliminate common problems such as paper jam and second transfer defect caused by setting ahead the timing for starting the polarity reversal.